The invention relates to an electron microscope provided with an electron source for the generation of a beam of electrons; an energy-dispersive element for the dispersion of the paths of electrons having a different kinetic energy; an accelerating tube for the acceleration to a predetermined beam energy of the electron beam from the electron source to a specimen to be studied with the aid of an electron microscope; a plate mounted between the energy-dispersive element and the specimen, in which a selection slit is provided at right angles to the dispersive direction of the dispersive element for the selection of dispersed electrons having a kinetic energy within a desired energy interval; and source imaging electron optics for obtaining an image of a source in the plane of the plate comprising the selection slit.
Such an electron microscope is known from practice. Generally speaking, with an electron microscope it is possible to see smaller structures than is possible with light, making use of the wave characteristics of the electrons to produce an image of the specimen, and of the extremely short wavelength of the electrons, allowing the observance of the small structures. An electron microscope further makes it possible to analyze the specimen by determining the transmission of electrons through the specimen. It is also possible to analyze the specimen by measuring the energy loss of the electrons after they have passed through the specimen. In general this energy loss is an extremely small fraction of the kinetic energy of the electrons in the beam. When using this method of determining the energy loss, it is possible to scan the specimen with a sharply defined electron beam by means of a point by point analysis of the specimen. The smaller the diameter of the beam on the specimen, the better the resolution of the analysis. The above summary is not exhaustive but merely gives an impression of the possibilities and the great importance of an electron microscope. For the various possibilities diverse types of electron microscopes, such as the transmission electron microscope (TEM) and the scanning electron microscope (SEM), have been developed.
The demands on the analytical properties and the spacial resolution of the various electron microscopes are continuously increasing. A possibility of improving them is to introduce an energy-dispersive element in the beam""s path from the electron source to the specimen. This makes it possible to reduce the electron""s energy dispersion in the beam, which dispersion is mainly caused by the different velocities with which the electrons leave the electron source. The reduction of this energy dispersion makes it possible to not only increase the energy resolution of energy loss spectra to be determined, but also to improve the spacial resolution of the microscope, since the setting parameters of the electron optics used will only be optimal for a specific kinetic energy of the electrons. To operate the electron microscope properly, the setting parameters have to be aligned optimally.
One difficulty when using the energy-dispersive element is that faulty imaging of the electron source may occur. For example, an astigmatic image may be formed, which means that there are different focusing planes for the dispersive and the non-dispersive direction of the dispersive element. The image of the electron source has to lie in the plane of the plate with the selection slit which is ultimately used for the selection of the electron beam""s appropriate energy interval. Such a plate may, for example, consist of two plate halves between which the slit is formed. In order to render the image stigmatic, it is possible to add a so-called stigmator to the source imaging electron optics. Such a stigmator may be integrated with the energy-dispersive element. To make the image stigmatic and to focus it in the plane of the plate comprising the selection slit, the setting parameters of the stigmator and a number of other elements of the source imaging electron optics also have to be optimized.
The specific embodiments of the various elements applied will not be entered into. Such elements are known in the electron microscopy and they use setting voltages and currents as setting parameters.
The optimization of these setting parameters poses a problem with the known electron microscope. When aligning this microscope, the electron source is imaged as well as possible in the plane of the selection slit, and a magnified image is subsequently projected onto the fluorescence screen of the electron microscope. However, it is not possible to assess whether the source is imaged properly in the plane of the selection slit, as this image is observed via a further image on the fluorescence screen. Another disadvantage is that the electron microscope""s magnifying optics for imaging the first image on the fluorescence screen have to be aligned separately, which is time consuming and cannot be carried out automatically.
It is the object of the invention to provide an electron microscope of which the setting parameters in respect to various optical elements for the electron beam can be aligned fully and efficiently. To this end the invention provides an electron microscope, characterized in that in addition to the selection slit, the plate comprises a plurality of further apertures useful for the determination of the cross-sectional form of the beam; and in that the electron microscope comprises means for the determination of the intensity of the beam being transmitted through and/or onto the plate so that subject thereto, setting parameters of the energy-dispersive element and the source imaging electron optics can be aligned.
The apertures in the plate allow the beam to pass in a particular crosswise direction with respect to the direction of the beam, with the shape of the apertures having to be such that the beam""s intensity can be determined in different cross directions. Said transmitted beam intensity can be measured by the means mentioned, so that said intensity coupled to a particular direction is known. In this manner immediate information is obtained regarding the dimensions and the position of the image in the plane comprising the selection slit. This is extremely important for the precise alignment of the setting parameters for obtaining an optimal image of the electron source in the plane of the selection slit.
The invention further provides the advantage that the optimization of the setting parameter can be performed automatically by interposing a calculation and control unit with an algorithm controlling the setting parameters in accordance with the particular beam intensities. An automatic optimization process further contributes to obtaining an optimal and quick alignment of the various setting parameters.
Preferably the membrane is positioned directly after the energy-dispersive element and the source imaging electron optics. This manner of positioning produces an image on the membrane that is not influenced by elements that do not need to be optimized, which results in the most precise alignment.
In a preferred embodiment the electron microscope is characterized in that the dimensions of the apertures are in the nanometric range; and in that the plate constitutes a thin membrane placed at a position where the electron""s kinetic energy is so low that it can be blocked by the thin membrane. Such small structures ensure that the obtained information relating to the location of the image in the plane of the selection slit has a high resolution, which further contributes to an extremely precise optimization. With such small structures it is necessary that the membrane they are made into is thin, having a thickness in the order of the dimensions of the apertures. Too thick a plate results in long and narrow channels through the plate. Such channels cause electrons to be scattered on their walls, which must be avoided as much as possible to prevent imaging problems.
It should be noted that from a contribution by the inventors at the EMAG ""97 conference (Electron Microscopy Analysis Group Conference, Cambridge, UK, 1997) placing a membrane having a narrow slit directly after an energy-dispersive element is already known. However, the considerations that culminated in the present invention of applying apertures with dimensions in the nanometric range and placing them directly after the elements to be optimized, are specific for the optimization of the alignment of their setting parameters.
To facilitate the alignment, a favourable embodiment is characterized in that the plate is permanently positioned, and in that the source imaging electron optics comprise at least one deflection means allowing the electron paths to be deflected in both the dispersive direction of the dispersive element and a direction perpendicular thereto. Such an embodiment is especially suitable for automatization.
The different apertures may have various shapes. In a preferred embodiment the further apertures comprise at least one additional slit at an angle with the selection slit, so that the intensity of the beam can also be determined in the dispersive direction, for instance, to make it possible for the image in the non-dispersive direction to also be projected in the plane of the selection slit.
In order to be able to determine the form of the beam in the dispersive direction only, the additional apertures comprise at least one slit parallel to the dispersive direction of the dispersive element.
For the composition of different configurations of slits the additional apertures comprise at least one slit parallel to the selection slit. A number of slits may be arranged in a star shape with the beam during optimization being able to move about the centre point of said star configuration, and in a pattern comprising at least one rectangle.
The other apertures further comprise preferably at least one opening much smaller than the cross-sectional dimension of the electron beam so that an exact image of the beam can be obtained, and so that only a small part of the image of the electron source is available when measuring specimens, while the edge effects of the source are blocked.
At the same time, the other apertures may comprise at least one opening which is much larger than the cross-sectional dimension of the electron beam in order to allow the whole beam to pass, or in order to obtain a slowly integrating signal when the beam is being moved from the plate into the opening.
In a preferred embodiment similar apertures are provided having different dimensions, so that openings can be selected whose dimensions, for example the width of the slit, match a desired energy interval or whose dimensions correspond with the size of the image of the electron source, to obtain a transmitted beam of maximum brightness.
In possible embodiments of the electron microscope according to the invention, the means for the determination of beam intensity transmitted through and/or onto the plate comprising the apertures include a current detector which, viewed in the direction of the beam, is placed after the plate and connected thereto is a current meter for measuring the electrons passing through the apertures; and/or a current detector which, viewed in the direction of the beam, is placed before the plate and connected thereto is a current meter for measuring the electrons reflected by the plate; and/or a current detector connected with the plate for measuring the incidence of electrons onto the plate, but not reflected by the plate.